Second Normal Form (2NF)
Second Normal Form:
Before we learn about second normal form, we need to understand the following:
Prime attribute: an attribute, which is part of prime-key, is prime attribute.
Non-prime attribute: an attribute, which is not a part of prime-key, is said to be a non-prime attribute.
Second normal form says, that every non-prime attribute should be fully functionally dependent on prime key attribute. That is, if X → A holds, then there should not be any proper subset Y of X, for that Y → A also holds.
[Image: Relation not in 2NF]
We see here in Student_Project relation that the prime key attributes are Stu_ID and Proj_ID. According to the rule, non-key attributes, i.e. Stu_Name and Proj_Name must be dependent upon both and not on any of the prime key attribute individually. But we find that Stu_Name can be identified by Stu_ID and Proj_Name can be identified by Proj_ID independently. This is called partial dependency, which is not allowed in Second Normal Form.
[Image: Relation in 2NF]
We broke the relation in two as depicted in the above picture. So there exists no partial dependency.